As technology nodes continue to decrease in critical dimensions, the number of photoresist strip steps increases along with the requirement for no material loss in the process. Photoresist is used in a variety of processes such as lift-off, thick dry films for plating, and protection for implantation to name a few. In order to withstand these process conditions, the photoresist is sometimes crosslinked and hardened to make it stable during the subsequent steps. This hardening makes it more difficult to remove with conventional techniques.
Lift-off of metal and other materials result in a complete coating being formed over the photoresist as shown in the present figures. As will be appreciated, the photoresist structure can take can number of different forms in that the photoresist can be deposited in a number of different patterns. As described herein, one lift-off technique involves immersing the wafers in a solvent bath to dissolve the patterned resist. The path for the solvent penetration can vary depending upon the sidewall angle in the lift-off structure. For examples, FIGS. 1A-C shows metal lift-off structures with different sidewall angles. FIG. 1A shows a first metal lift-off structure 10 that is formed of a lift-off polymer (patterned photoresist) 20 with metal 30 being deposited thereon. The structure also includes a metal pattern 11 that is formed on the substrate (wafer). As shown in the figures, after the photoresist cleaning process, the metal pattern 11 remains (see FIG. 2C). The lift-off polymer 20 in FIG. 1A has a sidewall structure defined by a negative angle. In FIG. 1B, the lift-off polymer 20 has a sidewall structure defined by vertical angle (i.e., the sides of the polymer 20 are vertical and parallel to one another). In FIG. 1C, the lift-off polymer 20 has a sidewall structure defined by a positive angle. It will be appreciated that the profile angle of the lift-off polymer 20 affects the shape of the deposited metal and the ease of the lift-off (with a positive angle sidewall structure (FIG. 1C) being difficult and a negative angle sidewall structure (FIG. 1A) being easier). Increasing the thickness of the lift-off polymer 20 assists in the removal of the lift-off polymer since it allows larger areas for solvent penetration. In some cases (e.g., positive angle sidewall as in FIG. 1C) there is complete coverage of the sidewall and therefore it is virtually impossible for the solvent to penetrate the lift-off polymer 20 for removal thereof.
FIGS. 2A-C illustrates solvent penetration and removal of the lift-off polymer (photoresist) 20 that has a sidewall structure according to one configuration. In this case, the lift-off polymer 20 has a negative angle sidewall structure as in FIG. 1A. FIG. 2A shows solvent penetration as a result of the wafer being exposed to solvent as by immersing the wafer within a solvent bath. The solvent penetrates the underlying photoresist (lift-off polymer 20) which results in the photoresist swelling and breaking apart. The photoresist dissolves in the solvent. FIG. 2B shows that the flow of solvent removes residual photoresist and acts to “lift” the deposited metal 30 from the surface. FIG. 2C shows the intended result of the process in that a clean metal pattern remains. In other words, the metal that does not overlie the photoresist and is arranged according to a predetermine pattern remains. It will be appreciated that the force required to completely remove photoresist depends upon several factors including but not limited to: the solubility of the photoresist, thicknesses of the photoresist and metal, and lift-off structure angle.
FIGS. 3A and 3B show one challenge to the photoresist removal process and in particular, during some manufacturing processes, a crust is formed on the top and sidewalls of the photoresist 20. More specifically, implantation for doping of the silicon result in an ion-rich layer and carbonized crust formed on the top and sidewalls of the photoresist 20 (solvent-rich photoresist layer). This crust 40 makes it difficult to penetrate the soft photoresist underneath and thereby strip. In FIG. 3A, an ion-induced carbonized layer 40 is formed on the photoresist 20 and an ion-rich layer or region 42 is formed on the photoresist 20. Implant sputter residues and implant species that are formed are indicated at 44.
FIGS. 4A-C depicts three conventional prior art methods for removing implanted resist. FIG. 4A depicts an undercut method for removing implanted resist 20 from a substrate (wafer) 15 (formation of the undercut facilitates removal of the resist 20); FIG. 4B depicts a method using mechanical force to remove implanted resist 20 from the substrate 15 (the mechanical force is used to penetrate (break apart) the crust layer and subsequently remove the resist 20); and FIG. 4C depicts a dissolution method for removing implanted resist 20 from the substrate 15. Unfortunately, none of these methods (techniques) are completely successful and therefore additional techniques are required in order to completely remove implanted resist 20.
Photoresist strip processes are typically a batch soak or single wafer spray process. However due to the impenetrable layer on top of the photoresist, the conventional processes, including a combination of these techniques, do not always provide complete photoresist removal.
There is therefore a need for an improved apparatus (system) and method for complete removal of the photoresist.